1. Field of the Invention
The present invention relates generally to material handling mechanisms whereby transfer of a material is effected from a transferring station to a receiving station, the rate of material transfer being responsive to a detected parameter at the receiving station, and more particularly, but not by way of limitation, to an improved paving material distribution system in combination with a paving machine used for paving a continuous lane along a roadway.
2. Description of the Prior Art
Traditionally, pavers laying bituminous material or the like operated by first laying the paving material on a prepared roadway, and then compressing the paving material by the use of a heavy roller. In recent years, machines have been developed that spread the material along the roadway and then provide a preliminary compression of the material with compression equipment mounted on or behind the spreader apparatus. Machines of this type are taught, for example, in the patents issued to Pollitz, U.S. Pat. No. 2,757,588; Bohannan, U.S. Pat. No. 2,591,502; and Pollitz, U.S. Pat. No. 2,911,892.
Further refinement has been brought about by the use of means to control a compressing screed that is pulled behind the powered spreading portion of the paving machine. For example, reference may be made to the patents issued to Shea, U.S. Pat. No. 3,264,959; Shea, U.S. Pat. No. 3,029,716; and Ackerman, et al, U.S. Pat. No. 3,236,163.
A recurring problem with the referenced types of paving machines has been that of spreading the paving material in a controlled and regulated manner. That is, paving machines of the type referenced generally have a hopper device near the front of the machine that receives paving material from a delivery truck or the like. A transfer mechanism, usually a slat conveyor, moves the material rearwardly where a screw conveyor spreads the paving material transversely to the roadway being paved, directly in front of a compressing screed. Since the quality of the compressed mat or lane is dependent on several factors, among which is the amount of paving material delivered immediately in front of the screed, it has been determined that the paving operation is highly improved by the maintenance of a constant volume of paving material in front of the screed.
To understand the reason for this, one must understand the action of the floating screed, which is a heavy planar member pulled over the transversely spread paving material. The screed is pulled against a mound of paving material that is continually formed by the deposited material in the path of the screed; the weight of the compressing screed is counteracted by the mound of paving material exerting back pressure on the screed which causes the screed to attempt to rise. If the amount of the material varies in front of the screed, the back pressure on the screed will vary, and the screed will alternately rise and fall, leaving in its wake a wavy and, often times, unacceptable paved surface. During the paving of even very smooth roadway bed surfaces, the maintenance of a constant screed back pressure is difficult because slight irregularities in the grade are reflected in the responsive movement of the screed. Such irregularities result in undulations in the finished pavement surface. It will be appreciated that when the grade is highly irregular, the paving machine operator has a very great problem in controlling the paving material feeding mechanism in a manner that effectuates a constant volume of paving material in front of the screed in order to achieve a uniform screed back pressure. This is true because the depressions encountered in the grade must be filled while high spots in the grade require less material, resulting in a variable amount of paving material required to form the mound in front of the screed. It should be readily appreciated that the skill of the paving operator is a very important consideration when paving with bituminous material or the like over irregular grade.
The Pollitz patent, U.S. Pat. No. 2,911,892, teaches the use of a screw conveyor in order to spread the paving material across the roadway in front of the screed, and this patent teaches the use of a sensor arm that contacts the outer edge of the material deposited by the screw conveyor in front of the screed. An off-on switch is controllably actuated by the position of the movable sensor arm, causing the clutch of the spreading screw conveyor to disengage in order to stop the rotation of the screw conveyor when sufficient material is sensed at the outer edge of the deposited material. The patent to Martinson, U.S. Pat. No. 3,678,817, teaches a further improvement in that the sensing paddle contacting the outer edge of the deposited material is part of a proportional control electrical circuit whereby the amount of material delivered to the sides of the machine is proportional to the amount of material required to supply the demand under the screed to effectuate constant back pressure. That is, the position of the sensing paddle generates a signal to control the rate of flow of the material delivered in front of the screed by the conveyors. A potentiometer is responsive to the position of the sensor paddle, thereby providing a variable electrical signal.
The present invention presents a further improvement over the prior art devices, in that a direct linkage, proportional control system is disclosed and claimed. By the use of direct mechanical linkages and rugged hydraulic valving, the present invention provides an improved material transfer mechanism that is useful in combination with a continuous paving machine to effectuate constant screed back pressure, resulting in the production of a superior paved surface when the paving machine is operated over variable grade roadbed surfaces.